Because of its small and well-characterized nervous system and amenability to genetic manipulation, the nematode Caenorhabditis elegans offers the promise of understanding the mechanisms underlying a whole animal's behavior at the molecular and cellular levels. In fact, this goal was a primary motivation behind the development of C. elegans as an experimental organism 40 years ago. Yet it has proven surprisingly difficult to obtain a mechanistic understanding of how the C. elegans nervous system generates behavior, despite the existence of a 'wiring diagram' that contains a degree of information about neural connectivity unparalleled in any organism. This review describes three types of information--molecular data on cellular neurochemistry, temporal information about neural activity patterns, and behavioral data on the consequences of neural ablation and manipulation--that, along with genetic analysis, may ultimately lead to a complete functional map of the C. elegans nervous system.